1. Field of the Invention
The present invention relates to a light emitting device, and more particularly, to an AC light emitting device, wherein a plurality of light emitting cells formed on a substrate are flip-bonded to a submount to be driven under an AC power source.
2. Discussion of the Background
With the development of GaN based light emitting diodes (LEDs), the GaN based LEDs have considerably changed LED technologies. Currently, the GaN based LEDs are used for various applications such as a full-color LED display, an LED traffic light, a white LED, and the like. Recently, it has been expected that high-frequency white LEDs will substitute for fluorescent lamps. In particular, efficiency of white LEDs has reached the level similar to that of typical fluorescent lamps.
In general, an LED emits light by a forward current and requires the supply of a DC current. Hence, when the LED is connected directly to an AC power source, it is repeatedly turned on/off depending on the direction of a current. As a result, there is a problem in that the LED does not continuously emit light, and is easily broken by a reverse current.
To solve such a problem, an LED capable of being connected directly to a high-voltage AC power source has been disclosed in PCT Patent Publication No. WO 2004/023568(A1), entitled “LIGHT-EMITTING DEVICE HAVING LIGHT-EMITTING ELEMENTS” by SAKAI et al.
According to PCT Patent Publication No. WO 2004/023568(A1), LEDs (light emitting cells) are two-dimensionally connected in series on a single insulative substrate such as a sapphire substrate, thereby forming an LED array. Such two LED arrays are in reverse parallel on the sapphire substrate. As a result, there is provided a single chip light emitting device, which can be directly driven by an AC power supply.
However, in the single chip AC light emitting device, since a plurality of light emitting cells connected in series to one another simultaneously operate, a considerable amount of heat is generated, and such generation of heat causes the light emitting efficiency of the light emitting cells to be degraded. Therefore, an AC light emitting device capable of dissipating heat generated from a chip is required.
Meanwhile, when LED arrays are connected in reverse parallel and operate under AC voltage, excessive reverse voltage is applied to light emitting cells in the array to which reverse voltage is applied, and thus, the light emitting cells may be broken down. FIGS. 26 and 27 in PCT Patent Publication No. WO 2004/023568(A1) disclose that the light emitting cells corresponding to each other provided in the respective LED arrays connected in reverse parallel share a negative electrode. Since the light emitting cells share the negative electrode, the same voltage as the voltage applied to the light emitting cells in the array to which forward voltage is applied is applied to the light emitting cells in the arrays connected in reverse parallel. Thus, it is possible to prevent overvoltage from being applied to the light emitting cells in the array to which reverse voltage is applied.
However, since the light emitting cells corresponding to each other provided in the respective arrays connected in reverse parallel share the negative electrode in PCT Patent Publication No. WO 2004/023568(A1), processes for forming the light emitting cells on a substrate, e.g., patterning processes, are complicated.